Only 10 t-Shirts have been produced and they form the prize for the 15th Anniversary competition that is being held on Sonic Channel. As with most Sonic events, the competition appears to be only available to Japanese residents.
The item has been named the E3 T-shirt to tie it in with the forthcoming release of next-gen Sonic the Hedgehog, even though the game wont actually be released until later on in the year.

More information can be found at Sonic Channel.